I couldn't get the date_time.xml to validate, however, it works anyway.
The build process with fail with adequate error messages to fix
any errors.

/********************************/
Do not place a dtd declaration in the doc xml files.
Begin each file with:
<section ...

/********************************/
Including files into sections has some quirks to it. Unless there is a 
section in the parent file, the first included file will display
instead of link.

Example.
In date_time.xml:

  <xi:include href="subsection.xml" />

And in subsection/xml:
  <section id="date_time.my_subsection"
     xmlns:xi="http://www.w3.org/2001/XInclude">
     <para>
       ...
     </para>
     <xi:include href="another.xml" />

/********************************/
To crosslink use:
<link linkend="date_time.section.subsection">optional text</link>

If you use '<link linkend="top">top</link>' the link will automatically
go to the top of the page it ends up in.